Stock-Based Compensation (Tables)

6 Months Ended

Jun. 30, 2013

Fair Values of Employee Stock Options

The estimated grant date fair values of the employee stock options were calculated using the Black Scholes valuation model, based on the following assumptions:

	ThreeMonthsEndedJune30,						SixMonthsEndedJune30,
	2013			2012			2013			2012
Risk-free interest rate		1.2	%		1.2	%		1.1	%		1.2	%
Expected life		6.0years			6.0years			6.0years			6.0years
Volatility		79	%		70	%		79	%		70	%
Dividend yield		—			—			—			—

Summary of Option Activities

The following table summarizes option activity under the 2003 Plan and the 2013 Plan and related information during the six months ended June30, 2013:

	SharesAvailable for Grant			SharesSubjectto Outstanding Options			Weighted-Average ExercisePricePer Share
Balance at December31, 2012		593,011			3,451,178		$	6.35
Options authorized		150,000			—			—
Options granted		(560,854	)		560,854			13.98
Options exercised		—			(72,204	)		3.52
Options cancelled		151,913			(151,913	)		7.33
Balance at June30, 2013		334,070			3,787,915		$	7.50

Classification of Stock-Based Compensation Expense

The table below sets forth the functional classification of stock-based compensation expense, net of estimated forfeitures, for the periods presented (in thousands):

	ThreeMonthsEndedJune30,				SixMonthsEndedJune30,
	2013		2012		2013		2012
Research and development	$	609	$	358	$	945	$	749
General and administrative		601		310		978		662
Total stock-based compensation	$	1,210	$	668	$	1,923	$	1,411